1. Field of the Invention
The present invention relates to a method for performing roaming in a dual mode mobile terminal, which can minimize a ping-pong phenomenon occurring under a weak electric field between communication networks providing communication services using different mobile communication techniques.
2. Description of the Related Art
Recently, a code division multiple access (CDMA) communication system has been implemented over a large area with a sufficient infrastructure. However, a third generation network, such as a wideband code division multiple access (WCDMA) network, has been gradually introduced in order to provide services which cannot be supported by a second generation network such as the CDMA network. In addition, a dual mode mobile terminal designed to receive a service specialized for the third generation network in an area located in the third generation network while using the infrastructure of the second generation network has been developed.
Recently, a dual mode user equipment or a dual mode mobile terminal simultaneously supporting the WCDMA and the CDMA has been developed out of necessity to provide a user with a mobile terminal capable of operating through both techniques (the WCDMA and the CDMA). One dual mode mobile terminal including the two modes requires various techniques capable of operating in the two modes. Among these techniques, one includes roaming being performed when the WCDMA mode and the CDMA mode are switched to each other inside of the terminal.
Generally, the roaming function enables a dual mode mobile terminal to perform switching between the CDMA mode and the WCDMA mode, so that the dual mode mobile terminal operating in the WCDMA mode in the WCDMA service area can switch from the WCDMA mode to the CDMA mode when the dual mode mobile terminal moves to a CDMA service area. The roaming function in an area where the typical CDMA system and the typical WCDMA system are installed will be described with reference to FIG. 1. FIG. 1 illustrates an area where the typical CDMA system and the typical WCDMA system are installed. The area, where a CDMA system and a WCDMA system are installed, is classified into a single CDMA area supporting only a CMDA service and a superposition area simultaneously supporting a WCDMA service as well as the CDMA service.
Generally, when a dual mode mobile terminal 10 set to a WCDMA preferred mode is turned on, the dual mode mobile terminal 10 first attempts to acquire the WCDMA network system. If the dual mode mobile terminal 10 successfully acquires the WCDMA network system, the dual mode mobile terminal 10 operates in the WCDMA mode. If the dual mode mobile terminal 10 fails to acquire a WCDMA network system, the dual mode mobile terminal 10 attempts to acquire a CDMA network system.
Thereafter, if the dual mode mobile terminal 10 having successfully acquired the WCDMA network system operates as the WCDMA mode in the WCDMA network, or in a superposition area 30 and then enters a single CDMA area 40, the dual mode mobile terminal 10 searches for a CDMA base station because it cannot detect a WCDMA base station. Then, if the dual mode mobile terminal 10 detects the CDMA base station, it completes a roaming process by switching from the WCDMA mode to the CDMA mode. In contrast, if the dual mode mobile terminal 10 enters the superposition area 30, the dual mode mobile terminal 10 in the CDMA mode performs roaming in order to switch from the CDMA mode to the WCDMA mode according to a predetermined condition. As described above, the dual mode mobile terminal 10 having acquired one network system attempts roaming to another network system if the intensity of a signal received from a network system, which covers the dual mode mobile terminal 10, is lowered to a predetermined intensity during a predetermined time interval after the acquisition.
Generally, when the dual mode mobile terminal 10 operating in the WCDMA mode exits the WCDMA service area, the dual mode mobile terminal 10 performs roaming to the CDMA mode. As described above, the dual mode mobile terminal 10 can determine, based on system parameters received from the CDMA base station, when the dual mode mobile terminal 10 is positioned in the superposition area capable of providing a WCDMA service or in an area supporting only a CDMA service. The determination may be achieved even when the dual mode mobile terminal 10 is in the CDMA mode. Accordingly, if it is determined that the dual mode mobile terminal 10 is positioned in the WCDMA superposition area 30 based on the system parameters received from the CDMA base station, the dual mode mobile terminal 10 operating in the CDMA mode performs roaming to the WCDMA network system.
In detail, as shown in FIG. 1, the dual mode mobile terminal 10 is positioned on a border cell 20 adjacent to the CDMA network in the WCDMA area. In this case, if the dual mode mobile terminal 10 receives system parameters, which are used for reporting entry into the superposition area 30, from the CDMA base station on a border between the superposition area 30 and the single CDMA area 40, the dual mode mobile terminal 10 performs handover toward the WCDMA network. In other words, the dual mode mobile terminal 10 communicates with the WCDMA network. However, if it is determined that signal intensity of the WCDMA network in a shadow area 50 is included in a weak electric field (e.g., inside of the building) having at most a preset threshold value, the dual mode mobile terminal 10 continuously attempts roaming until the dual mode mobile terminal 10 acquires the WCDMA network system in a state where the dual mode mobile terminal 10 has acquired the CDMA network system. Accordingly, as the dual mode mobile terminal passes through the shadow area 50, which is an area having a weak WCDMA receive signal intensity, a ping-pong phenomenon occurs in which a handover procedure is repeated by continuously performing the roaming function from the CDMA network 40 to the WCDMA network 30.
As described above, conventionally, when a dual mode mobile terminal attempts roaming from one mode to another mode in a weak electric field area, if the one mode is not switched to another mode corresponding to a network (a target of roaming), the mode of the dual mode mobile terminal returns to the one mode, this back-and-forth process is referred to as a “ping-pong phenomenon”. As described above, when roaming is repeatedly performed between different networks according to the intensity of a received signal; the probability of errors of the dual mode mobile terminal may increase. In addition, when full-searching is performed to search for cells in the WCDMA service area, the dual mode mobile terminal remains in a system switching state during the range of 20 seconds to a maximum of 2 minutes. Accordingly, the dual mode mobile terminal does not receive services during the system switching state, thereby causing an inconvenience to a user for receiving the communication service.